The present invention relates to a thermal head suitable for color printers or the like, a substrate used in the thermal head, and an image recording method.
A structure of a single-line thermal head comprising a plurality of exothermic resistors formed in a line will be described, with reference to FIG. 1 and FIG. 2. FIG. 1 is a perspective view of a single-line type thermal head, and FIG. 2 is a sectional view of the thermal head, taken along the line A-Axe2x80x2 in FIG. 1. In these figures, reference symbol 101 denotes an alumina substrate, and on the upper face of this substrate 101, there are formed respective parts of the thermal head, and on the bottom face thereof is adhered a radiation fin 102. The radiation fin 102 is for efficiently radiating heat generated in each part in the operation of the thermal head, into the air.
Reference symbol 103 denotes exothermic resistors, which generate heat when an electrical current is made to flow between a common electrode 104 and individual lead electrodes 105. The common electrode 104 is an electrode common to all exothermic resistors 103, and is connected respectively to contact portions 106 of each exothermic resistor 103. The individual lead electrodes 105 are connected to each contact portion 107 of each exothermic resistor 103, and wired respectively to each terminal 109 of an IC (Integrated Circuit) 108.
Reference symbol 110 denotes a glaze, formed in a half spindle shape on the upper face of the alumina substrate 101, and functions as a heat reservoir for storing heat energy generated by the exothermic resistor 103 at the time of printing processing. Reference symbol 111 denotes a flexible printed circuit board for connection, and a wiring for connecting with a controller of a printer body (not shown) is formed thereon. Reference symbol 112 denotes a protection layer, for protecting the exothermic resistor 103 and electrodes 104, 105 from wear due to contact with the paper at the time of printing.
A production method of the thermal head in FIG. 1 will now be described. At first, in order to remove dust on the surface of the alumina substrate 101, the alumina substrate 101 is cleaned. After cleaning, a thin film of the exothermic resistors 103 is formed by sputtering using a sputtering system, on the upper face of the alumina substrate 101, so that the exothermic resistor film has a predetermined sheet resistance. An electrode material (for example, aluminum) is then formed on the upper face of the thin film material of the exothermic resistors 103 by sputtering or a vapor deposition method.
A photoresist is then coated on the electrode material film, to thereby prepare a resist pattern of the common electrode 104 and the individual lead electrodes 105 by photolithography. The electrode material is etched using this photoresist pattern as a mask, to form the common electrode 104 and the individual lead electrodes 105. The whole resist is then removed, and a new resist is coated on the thin film material of the exothermic resistors 103, the common electrode 104 and the individual lead electrodes 105.
Then, a resist pattern for forming the exothermic resistors 103 for each printing dot is formed by photolithography. A thin film consisting of the exothermic resistors 103 is divided into exothermic resistors 103 for each dot by etching. A protection film 112 is then formed on the upper part of the glaze 110 by sputtering, using a mask for forming the protection film. Then, the protection film 112 is subjected to a heat treatment, for realizing stabilization of a resistance value of the exothermic resistors and stabilization of intimate contact between the exothermic resistors and the electrode material.
An insulating film is formed in the IC mounting area, and an IC 108 is subjected to die bonding on this IC mounting area Terminals of the IC 108 and wire-bond terminals 109 of the individual lead electrodes 105 are connected by wire bonding, and seal the IC 108, the wire bond portion and a part of the individual lead electrode 105 are sealed by a resin. A single-line thermal head is produced by the above-described production process.
As a second conventional example, there is shown a thermal head in FIG. 3 and FIG. 4 (see Japanese Patent Application No. 62-217627). FIG. 3 is a plan view of a double-line thermal head where a plurality of exothermic resistors are arranged in two lines in parallel, and FIG. 4 is a sectional view, taken along the line B-Bxe2x80x2 in FIG. 3. As shown in these figures, a first alumina substrate 301 and a second alumina substrate 302 are connected with a metal plate 314 placed therebetween. The metal plate 314 is a common electrode and connected with other common electrode 313.
Reference symbol 305 denotes a first exothermic resistor, and is connected to a first individual lead electrode 306 via a contact area 307, and is connected to a common electrode 313 via a contact area 312. A second exothermic resistor 309 is connected to a second individual lead electrode 315 via a contact area 316, and is connected to a common electrode 313 via a contact area 310. Reference symbol 311 denotes a protection layer, which protects the exothermic resistors 305 and 309 from wear due to contact with a sheet of paper to be printed.
As a conventional third example, there is a double-line thermal head having a section shown in FIG. 5. In this figure, a wiring groove 318 is formed in an alumina substrate 300, and a common electrode 317 is formed therein by embedding a bulk metal into the wiring groove 318. A common electrode 313 is formed on the wiring groove 318, and connected to the common electrode 317.
The operation of the thermal head shown in FIG. 1 will now be described with reference to FIG. 6. FIG. 6 shows an equivalent circuit of the thermal head, wherein reference symbol 120 denotes a power source, which supplies drive power for the thermal head. Reference symbol 103 denotes an exothermic resistor, 104 denotes a common electrode, 105 denotes an individual lead electrode, and 108 denotes a control IC.
At first, a data signal DATA corresponding to each exothermic resistor 103 is input at to the control IC 108, synchronized with a clock signal CLK having a constant period transmitted from a printer body (not shown), and information of the data signal DATA is stored in a storage section inside the control IC 108, upon xe2x80x9cbuild upxe2x80x9d of a latch signal LATCH. Based on the stored information, for example, when a strobe signal STB is xe2x80x9c1xe2x80x9d, the exothermic resistors 103 are energized to generate heat energy. Here, at the time of printing, printing information of the next line is transferred from the printer body synchronized with the clock signal CLK, by means of the data signal DATA. The control IC controls ON/OFF of the exothermic resistors 103 based on the data supplied from this control section. The thermal head substrate is secured to a heat sink 102 by means of double sided adhesive tape, adhesive or the like.
On the other hand, a heat sensitive paper made to develop color by the thermal head has a construction shown in FIG. 7. This heat sensitive paper has such a construction that a cyan recording layer 712, a magenta recording layer 713 and a yellow recording layer 714 are sequentially laminated on a base material 711 such as paper, and the surface is covered with a heat-resistant protection layer 715. The cyan recording layer 712 has a structure such that microcapsules 717 are dispersed in the cyan developer 716, and a cyan leuco dye 718 which reacts with the cyan developer 716 and makes it develop color is sealed in these microcapsules 717.
The magenta recording layer 713 has a structure such that microcapsules 720 are dispersed in the magenta recording layer 713 mainly composed of a coupler 719, and a magenta diazo dye 721 which reacts with the coupler 719 and develops magenta color is sealed in these microcapsules 720.
Moreover, the yellow recording layer 714 has a structure such that microcapsules 723 are dispersed in a yellow coupling agent 722, and a yellow diazo dye 724 which reacts with the yellow coupling agent 722 and develops color is sealed in these yellow microcapsules 723.
FIG. 8 shows one example of a conventional printer constructed in this manner and using a full color heat sensitive paper. Reference symbol 830 denotes a paper cassette, and in this paper cassette 830, heat sensitive papers 831 having the above-described construction are stacked. Above the heat sensitive paper 831 in the stacked condition, there is provided a feed roller 832 which is brought into contact with the upper face of the heat sensitive paper to exert a frictional force thereon in the direction of the page (in the rightward direction in FIG. 8), and a paper guide 833 is provided in the feed direction of the feed roller 832, to guide the heat sensitive paper upwards. Above the paper cassette 830, there is provided a belt 838 wound around rollers 834, 835, 836 and 837. Of these rollers 834 to 837, the roller 836 clamps and holds the heat sensitive paper with a roller 839, and feeds the heat sensitive paper in the direction of the arrow in the figure at a predetermined timing. The roller 837 is a platen roller and is disposed opposite to the thermal head 870.
On the periphery of the belt 838, there is provided a damper 839A for clamping the heat sensitive paper 831 fed out from the paper cassette 830, and the heat sensitive paper 831 is clamped and held by this damper 839A.
At a position on the downstream side of the thermal head 870, there are provided a Y lamp 840 and an M lamp 841 respectively for irradiating beams of light having a predetermined wavelength onto the surface of the heat sensitive paper 831. The operation of these lamps 840 and 841 will be described later. At a position on the further downstream side of the lamps 840, 841, a pair of paper ejection rollers 842, 843 are disposed in the vicinity of the, roller 834, so that the tip of the heat sensitive paper which tends to move in the tangent direction away from the belt 838 bent around the roller 834 is clamped and held therebetween and ejected. On the outer periphery of the other paper ejection roller 842 is disposed a paper guide 844, which guides the printed heat sensitive paper fed out from the roller 842 in a predetermined paper ejection direction.
The principle of color printing in the printer having the above-described construction will be described using FIGS. 7 to 10. The heat sensitive paper 831 whose tip is clamped and held by the damper 839A of the belt 838 is fed to the platen roller 837. At a timing when the tip of the heat sensitive paper 831 passes the platen roller 837, the thermal head 870 is pressed onto the heat sensitive paper 831, and processing comprising the following steps (a) to (e) is executed.
(a) As shown in FIG. 9A, when the yellow recording layer 714 is heated, the yellow capsule 723 therein is softened due to the heat, and the yellow coupling agent 722 penetrates into the yellow capsule 723 to thereby react with the yellow diazo dye 724 and develop color (shaded portion in the yellow recording layer 714 in FIG. 9A). The transmission quantity of the yellow coupling agent 722 is proportional to the energy quantity applied onto the heat sensitive paper 831 from the thermal head 870, as shown in FIG. 10, and color is developed in yellow concentration due to the property shown in FIG. 10, depending on the applied energy. Since the magenta capsule 720 and the cyan microcapsule 717 are set to have a higher softening temperature than that of the yellow microcapsule 723, the magenta recording layer 713 and the cyan recording layer 712 do not develop color.
(b) When the tip of the heat sensitive paper 831 reaches the position of the yellow fixing lamp (Y lamp) 840, as shown in FIG. 9B, the yellow fixing lamp 840 is lighted, to thereby decompose the undeveloped yellow dye by the light.
(c) As shown in FIG. 9C, the belt 838 is made to go around to feed the heat sensitive paper 831 again to the thermal head 870, to develop magenta color. Specifically, the magenta microcapsule 720 is softened by heat, and the magenta diazo dye 721 therein is reacted with the magenta coupling agent 719 to develop color (shaded portion in the magenta recording layer 713). The softening temperature of the cyan microcapsule 717 is set higher than that of the magenta microcapsule 720, and hence the cyan recording layer does not develop color. With regard to the transmission quantity of the magenta coupling agent 719, color is developed in a concentration proportional to the energy quantity applied onto the heat sensitive paper 831 from the thermal head 870, as with the case of yellow.
(d) As shown in FIG. 9D, when the tip of the heat sensitive paper 831 reaches the position of the magenta fixing lamp (M lamp) 841, the magenta fixing lamp 841 is lighted, to thereby decompose the undeveloped magenta dye by the light to lose the color development capability. The magenta fixing lamp 841 decomposes the magenta dye with beams of light having a peak at a wavelength of 365 nm.
(e) As shown in FIG. 9E, the belt 838 is made to go around to feed the heat sensitive paper 831 again to the thermal head 870, to develop cyan color. Specifically, the cyan microcapsule 717 is softened by heat, and the cyan leuco dye 718 therein is reacted with the cyan developer 716 to develop color (hatched portion in FIG. 9E).
When full color printing is completed by the cyan color development in the above step (e), the tip of the heat sensitive paper 831 is removed from the damper 839, and fed to between the paper ejection rollers 842 and 843, to thereby be ejected along the guide plate 844. According to need, the belt 838 may be further made to go around, to thereby perform bleach processing of the non-developed portion by the yellow fixing lamp 840 and the magenta fixing lamp 841.
On the other hand, if the double-line thermal head shown in FIG. 3 is used, two lines can be printed simultaneously, and hence the printing time can be reduced to half, in principle.
However, with the thermal head shown in FIGS. 3 to 5, since the coefficient of thermal expansion of the common electrode using a bulk metal and the alumina substrate is different, peeling easily occurs on the connection interface thereof. If peeling occurs between the alumina substrate and the bulk metal, thermal stress is applied to the thin film electrode formed on the common electrode, and since the mechanical strength of the thin film is low, the thin film suffers damage. As a result, there is a disadvantage in that implementation is difficult. A first object of the present invention is to provide a double-line thermal head which is practical and capable of high-speed printing.
On the other hand, the present inventors have proposed to utilize a double-line thermal head and use exothermic resistors in one line for preheating for applying bias energy. In this case, it is not necessary to control ON/OFF of the exothermic resistor for each unit of printing dot. That is to say, it is not necessary to connect each heating element individually to the control IC, and these may be ON/OFF controlled collectively, or put together in two or more blocks. A second object of the present invention is to provide a thermal head which can be produced at a low production cost, has a preheating function and is capable of high-speed printing.
Speed-up of the printing speed is also possible by a printing method using a plurality of thermal heads, other than by improving the thermal head. FIG. 11 shows a structure of a high-speed printer using three thermal heads, wherein on a color heat sensitive paper 1102 drawn out from a paper cassette 1101, yellow is developed by a yellow thermal head 1111Y, and undeveloped yellow dye is decomposed by a yellow fixing lamp 1121Y, then magenta is developed by a magenta thermal head 1111M, and undeveloped magenta dye is decomposed by a magenta fixing lamp 1121M, and further cyan is developed by a cyan thermal head 1111C, and undeveloped cyan dye is decomposed by a cyan fixing lamp 1121C. Thermal heads 1111Y-C are the same as those shown in FIG. 1 or FIG. 3.
According to this apparatus, since there are raised portions due to the thickness of the IC, as shown in the figure, on the thermal head substrate faces 1112Y-C, guide rollers are used to bend a path line for the sheet in a complicated shape, in order to avoid these raised portions. Accordingly, there is a disadvantage in that not only the mechanism is complicated, but also maintenance of positioning precision in each thermal head becomes difficult, and hence deviation in the printing dot easily occurs. A third object of the present invention is to provide a thermal head in which the path line for the sheet can be constructed straight.
In the energizing pulse length (energy) required for development of each color, there is the relationship as shown in FIG. 10.
pulse length of yellow less than pulse length of magenta less than pulse length of cyan
The marginal energy immediately before each color Y, M, C is developed is assumed to be bias energy PBY, PBM and PBC, as shown in FIG. 10. On the other hand, the energy required for representing a predetermined gradation for each color is denoted by PGY, PGM and PGC in FIG. 10 and at the time of actual development of color, pulses corresponding to PBY+PGY, PBM+PGM, and PBC+PGC are supplied to the thermal head 870.
Generally, physical properties are adjusted so that the maximum value of PGY, PGM and PGC in the heat sensitive paper 831 becomes substantially the same value.
PGY≈PGM≈PGC≈PGxe2x80x83xe2x80x83Expression (1)
(wherein PG=pulse length corresponding to the maximum gradation)
Also, in the case of a direct thermosensitive method, as is obvious from FIG. 10, the following relationship is established between each pulse:
PBM≈PBY+PGPBC≈PBM+PG≈PBY+2PGxe2x80x83xe2x80x83Expression (2)
Moreover, the net printing time PT is calculated by the following expression:
PT={(PBY+PG)+(PBM+PG)+(PBC+PG)}xc3x97number of linesxe2x80x83xe2x80x83Expression (3)
wherein PT is the net time required for printing three colors, and in the actual printing, a longer time than PT is required since paper taking-in and ejection time is included.
A fourth object of the present invention is to provide a printing method in which the energy required for development of colors is efficiently effected on the heat sensitive paper to thereby reduce the time required for printing and improve printing capability, in view of the above-described situation.
Furthermore, a fifth object of the present invention is to provide a printing mechanism which uses the aforesaid printing method to constitute a straight carrier path required for realizing high precision superposition of dots.
A substrate for a thermal head according to the present invention comprises: an exothermic resistor section in which exothermic resistors are provided; an IC mounting section on which an IC is mounted so as to energize the exothermic resistors; and a wiring section in which wiring is arranged to connect the exothermic resistor section and the IC mounting section; and a thickness of at least a part of the wiring section is smaller than that of the exothermic resistor section and the IC mounting section.
According to such a substrate for a thermal head, a thermal head can be manufactured such that the wiring section in which bending distortion does not become a problem is bent, but without bending the exothermic resistor portion and the IC mounting section where it is desired not to cause bending distortion. As a result, it becomes possible to prevent interference between an IC mounted in the IC mounting section and a heat sensitive paper, and to make the traveling route of the heat sensitive paper straight.
In the case of the double-line thermal head, two or more wiring sections are provided, and a thin portion may be formed in each of the wiring sections. Also, as the material of the substrate, metals such as iron alloy containing Ni and Al or stainless steel are preferable, but the material is not limited thereto.
On the other hand, the thermal head according to the present invention comprises a substrate, an insulating layer which is disposed on the substrate, with a raised portion being formed by raising a part of the surface thereof, and exothermic resistors formed on the raised portion, and a common electrode is disposed on the substrate, which protrudes from the surface of the substrate, penetrates through the raised portion and is connected to the exothermic resistors, to thereby divide the resistors into first exothermic resistors and second exothermic resistors, centering on the connecting point.
According to such a thermal head, after preheating of a heat sensitive paper is performed by the heating energy generated by the first exothermic resistors, at the time of printing, the heating energy of the second exothermic resistors is applied to effect the printing operation. Hence the energizing pulse of each exothermic resistor can be made short, thereby enabling reduction of the printing time.
Also, a thermal head according to the present invention may comprise a substrate on the central surface of which a common electrode portion having a predetermined length is protrudingly formed, a first insulating material formed on the surface of the substrate on one side of the common electrode portion, a second insulating material formed on the surface of the substrate on the other side of the common electrode portion, first exothermic resistors formed on the surface of the first insulating material with one end thereof being electrically connected to the common electrode portion, and second exothermic resistors formed on the surface of the second insulating material with one end thereof being electrically connected to the common electrode portion.
Moreover, the volume of the raised portion in the insulating layer surrounded by the first exothermic resistors and the common electrode may be formed larger than that of the raised portion in the insulating layer surrounded by the second exothermic resistors and the common electrode.
In this case, an effect can be obtained in that loss of the thermal energy generated by the first exothermic resistors is small, and is not affected by the amount of the thermal energy generated in the second exothermic resistors in printing of the next line, and thermal energy can be supplied as bias energy with high precision.
Furthermore, the raised portion in the insulating layer surrounded by the first exothermic resistors and the common electrode may be formed of a heat reserve material. In this case, since the raised portion in the insulating layer surrounded by the first exothermic resistors and the common electrode may be formed of a heat reserve material, heat can be transmitted to the heat sensitive paper efficiently. Hence heating energy generated by the first exothermic resistors can be efficiently used. The raised portion surrounded by the second exothermic resistors and the common electrode may be also formed of a heat reserve material.
The raised portion in the insulating layer surrounded by the first exothermic resistors and the common electrode may be formed thicker than other areas in the insulating film. In this case, since loss of the heat energy generated by the first exothermic resistors on the radiation fin (heat sink) side becomes small, an effect can be obtained in that the width of the energizing pulse to the second exothermic resistors can be made short.
The substrate is a metal substrate, and since this metal substrate and the common electrode are integrally formed, these have the same potential, and the metal substrate may have a function as an electrode. Moreover, the width of the common electrode in the traveling direction of the heat sensitive paper may be 2 mm or less.
Furthermore, the leads of the first exothermic bodies may be put together or united in a block unit and connected to a transistor. In this case, the number of transistors required is the same as the number of blocks. Also, in the above described thermal head, the second exothermic resistors may be provided ahead of the first exothermic resistors in the feed direction of the printing paper. When the first exothermic resistors are provided ahead of the second exothermic resistors in the feed direction of the printing paper, after the heat sensitive paper is heated to a threshold temperature immediately before developing color by the heat energy generated by the first exothermic resistors, the heating energy of the second exothermic resistors is added to thereby perform the printing operation. Hence, the energizing pulse to the second exothermic resistors can be made short, to thereby obtain an effect in that reduction of the printing time is possible.
On the other hand, the printing method according to the present invention is a method of developing color on a printing paper by heating it with exothermic bodies, characterized in that bias energy required at least for color development of the printing paper is given to the printing paper by first exothermic bodies, and then energy is applied by second exothermic bodies to a portion to be printed in the preheated portion to which the bias energy has been given, to thereby develop color on the printing paper in a desired gradation concentration. Thereby, the printing time can be shortened.
Moreover, a color printer according to the present invention comprises a heat sensitive paper on which a first coupler that develops a first color upon application of energy larger than a first color development energy, a second coupler that develops a second color upon application of energy larger than a second color development energy, and a third coloring material that develops a third color upon application of energy larger than a third color development energy are laminated and coated, a transport device which transports the heat sensitive paper in line units, and either one of the thermal heads described above, the surface of the thermal head is formed in a curved shape, and the thermal head is provided in the middle of a straight transport passage of the heat sensitive paper.